Field of the Invention
The invention relates to a device and a method for developing an electrostatic latent image, more particularly, one that is located on a movable image carrier, the latent image being developed by the use of a single or one-component toner.
Electrographic printing with high quality and at high speed is possible according to the present state of the art by two-component toners or one-component toners. A type of development of a latent image by a one-component toner has become known heretofore from the published German Non-prosecuted Patent Application (DE-OS) 197 28 309.
A main difficulty with one-component systems is that a uniform layer of toner particles which are charged as uniformly as possible have to be generated on a developing roller, also termed an ink applicator roller. Some commercially used systems employ a rejuvenator roller formed of a material resembling foam plastic, for transporting toner particles from a toner reservoir to the developing roller. Friction produced in this process charges the toner particles electrically, due to which they adhere to the electrically conductive developing roller in a more-or-less thick layer. In order to homogenize this layer, use has been made of fixed doctor blades which scrape off excess toner from the developing roller. There are systems with a hard developing roller, for example, formed of aluminum or steel, and a rubber lip as a doctor blade, as well as systems also with a hard doctor blade and a developing roller made of rubber material.
The hereinaforementioned German Non-prosecuted Patent Application (DE-OS) 197 28 309 describes an electrostatic developing device for one-component toners which, by comparison with the prior art of one-component toners, permits a clearly higher speed. In this case, the toner particles are initially applied to the developing roller via a so-called rejuvenator roller formed of plastic material. The friction produced during this process charges the toner particles electrostatically, and they adhere via an electrostatic imaging force to the surface of the metallic developing roller. In a subsequent step, a doctor roller serves to reduce the thickness of the toner layer to a desired thickness and to homogenize it or make it uniform. Speeds of at least 50 cm/sec. are possible with such a developing device. It is regarded as disadvantageous, with regard to the developing stations known in the prior state of the art, that the toner layers cannot yet be made ready on the developing roller with adequate uniformity, for the purpose of high-quality printing. With regard to relatively high speeds, in particular, in the doctor blade processes known from the prior state of the art, slight stripes are formed during the respective doctor blade process, and become quite distractingly apparent in the developed image.